In audio communications, audio quality may be degraded by hum noise interfering with audio signals due to a problem with a certain circuit such as an amplifier or an AD or DA converter (e.g., an amplifier circuit is not insulated from a power supply circuit).
In order to detect hum noise, an input signal may be converted from the time domain to the frequency domain, and the presence of hum noise at a predetermined hum noise frequency is detected when a stationary peak is present at this frequency. The predetermined hum noise frequency may be 50 Hz or 60 Hz and its harmonic overtones where 50 Hz and 60 Hz correspond to the frequencies of commercial power supply in Japan.
The frequency component may not form a peak at the frequency where hum noise is supposed to produce a peak due to the mixing of interfering sounds such as voices and background noises. In such a case, hum noise may not be detected at this expected frequency.
In the following, the above-noted problem will be described in detail. FIGS. 1A through 1C are drawings illustrating examples of cases in which hum noise is not detected. As illustrated in FIGS. 1A through 1C, when the spectrum of interfering sounds is superimposed on the spectrum of hum noise, a peak at frequency A disappears at the position where the hum noise is supposed to produce a peak. In this case, a num noise component is not detected at this frequency A. When hum noise components at other frequencies are removed, an unnatural voice sound may be obtained as a result.    [Patent Document 1] Japanese Patent Application Publication No. 2005-77423